Application of ROP improvement technology in middle and deep strata of the Bohai Sea

The principal reservoir of Bohai A Oilfield is the Archean buried-hill formation and it is lithologically composed of gneiss. The vertical depth of the target layer is over 4 000 m. A set of pebbled sandstones with pebble size over 7 mm are developed in the Kongdian Formation in the upper part. When the conventional PDC bit is used, its teeth are broken up seriously and it is replaced frequently. When the cone bit is adopted, it is wore seriously with low rate of penetration (ROP) and long drilling cycle, which extends the soaking time of open hole, leading to frequent collapse of the upper strata. As a result, the development of the oilfield is restricted seriously. In this paper, the formation drillability grade profile was determined by regressing the logging data and checking the core strength, so as to provide the theoretical basis for bit selection. Then, the advantages and disadvantages of various ROP improvement tools were analyzed. The integrated complex impact tool was adopted in the heterogeneous pebbled sandstones of Kongdian Formation, and consequently the early bit damage caused by tooth breakage was reduced. The torsional impact tool, combined with Ninja-tooth PDC bit was introduced into buried-hill gneiss formation, and consequently the ROP was improved effectively and the single-bit drilling footage is increased significantly. Thus, the improvement of ROP in middle and deep strata in the Bohai Sea is realized. To sum up, the bit selection technology for middle and deep strata in the Bohai Sea is developed in the exploration process of Bohai A Oilfield. This technology is verified in the drilling practice. It decreases the development threshold of middle and deep strata, cuts down the exploration and development costs, and provides the experience and technical support for the future development of middle- and deep-seated oil fields in the Bohai Sea.